The present invention relates to the fabrication of three-dimensional objects using additive process modeling techniques. More particularly, the invention relates to modeling machines which form three-dimensional objects in a heated chamber by depositing modeling material from a dispensing head onto a modeling base as the dispensing head and the base are moved in three-dimensions with respect to each other.
Additive process modeling machines make three-dimensional models by building up a modeling medium, usually in planar layers, based upon design data provided from a computer aided design (CAD) system. A mathematical description of a physical part to be created is split into (usually) planar layers, and those layers are individually shaped and applied to produce the final part. Three-dimensional models are used for functions including aesthetic judgments, proofing the mathematical CAD model, forming hard tooling, studying interference and space allocation, and testing functionality. The dominant application of layered manufacturing in recent years has been for rapid prototyping.
Examples of apparatus and methods for making three-dimensional models by depositing layers of flowable modeling material are described in Valavara U.S. Pat. No. 4,749,347; Crump U.S. Pat. No. 5,121,329, Batchelder, et al. U.S. Pat. No. 5,303,141, Crump U.S. Pat. No. 5,340,433, Batchelder, et al. U.S. Pat. No. 5,402,351, Crump et al. U.S. Pat. No. 5,503,785, Abrams et al. U.S. Pat. No. 5,587,913, Danforth, et al. U.S. Pat. No. 5,738,817, Batchelder, et al. U.S. Pat. No. 5,764,521 and Comb et al. U.S. Pat. No. 5,939,008, all of which are assigned to Stratasys, Inc., the assignee of the present invention. An extrusion head extrudes heated, flowable modeling material from a nozzle onto a base. The base comprises a modeling substrate which is removably affixed to a modeling platform. The extruded material is deposited layer-by-layer in areas defined from the CAD model, as the extrusion head and the base are moved relative to each other in three dimensions by an x-y-z gantry system. The material solidifies after it is deposited to form a three-dimensional model. It is disclosed that a thermoplastic material may be used as the modeling material, and the material may be solidified after deposition by cooling.
Technology described in the aforementioned patents is commercialized in Stratasys FDM® modeling machines. The extrusion head, which includes a liquifier and a dispensing nozzle, receives modeling material in a solid form. The filament is heated to a flowable temperature inside the liquifier and it is then dispensed through the nozzle. Thermoplastic materials, particularly ABS thermoplastic, have been found particularly suitable for deposition modeling in the Stratasys FDM® modeling machines. A controller controls movement of the extrusion head in a horizontal x, y plane, controls movement of the build platform in a vertical z-direction, and controls the feeding of modeling material into the head. By controlling these processing variables, the modeling material is deposited at a desired flow rate in “beads” or “roads” layer-by-layer in areas defined from the CAD model to create a three-dimensional object that resembles the CAD model. The modeling material thermally solidifies, and the finished model is removed from the substrate.
As a thermoplastic material cools, and particularly as it transitions from a flowable material to a solid, stresses caused by density changes of the thermoplastic (i.e., shrinkage) are generated in the material. These stresses can cause geometric distortion of a model. Accordingly, it is an objective in model building systems which employ thermal solidification to relieve the stresses caused by cooling, so as to minimize geometric distortion. Deposition materials other than thermoplastics, such as metals, thermoset polymers and composites share analogous challenges of minimizing geometric distortion produced by changes in density, shear, temperature and pressure associated with the extrusion process. As disclosed in U.S. Pat. No. 5,866,058, building the model in a chamber heated to a temperature higher than the solidification temperature of the thermoplastic or other thermally solidifiable modeling material, followed by gradual cooling, relieves stresses from the material. The stresses are annealed out of the model while is being built so that the finished model is stress free and has very little distortion. As is further disclosed in the '058 patent, the temperature of the chamber should be maintained below the glass transition temperature (Tg) of the modeling material, so that the model does not become so weak that it droops. The preferred temperature of the build chamber is in a range between the material's solidification temperature and its creep relaxation temperature (creep relaxation temperature is defined as the point at which the stress relaxation modulus has dropped by a factor of ten from its low temperature limit). In the case of ABS thermoplastic, the temperature window falls between approximately 70° C. and 90° C.
Existing Stratasys FDM® machines build models in a chamber (also referred to as a build envelope or oven) heated to between 70° C. and 90° C. The base on which the model is built is located in the heated chamber, as are the extrusion head and the x-y-z gantry. Placing the extrusion head and the x-y-z gantry in this heated environment has many disadvantages. The x-y-z gantry is comprised of motion control components, such as motors, bearings, guide rods, belts and cables. Placing these motion control components inside the heated chamber minimizes the life of these components. Additionally, the upper limit on the chamber temperature is constricted to a temperature at which the motion control components are operable. Such a limitation on the temperature of the chamber consequently limits the materials useful for modeling in the machine to those which will stress relieve at a relatively low temperature.
Similarly, placing the extrusion head in the heated chamber in the Stratasys FDM® machines required that a cooling mechanism be provided to cool the modeling material feedstock as it is supplied to the head in solid form (either as a filament or a wafer of material). A mechanism for cooling wiring harnesses is also provided. In the event of a power failure or power down, the material feedstock and the wiring harnesses that are normally cooled are exposed to the oven temperature. Finally, as a practical matter, in the event that adjustment, servicing, repair, replacement or of the motion control components or of the extrusion head are required, an operator must work inside the chamber. As such, the chamber must be cooled before these activities can be safely performed.